As is well known, demands for an optical storage system that can store a large amount of data, such as data for a motion picture film, have been increasing. Therefore, various types of holographic digital data storage system incorporating therein a storage medium have been recently developed for realizing high density optical storage capabilities.
The holographic digital data storage system allows a signal beam having information therein to interfere with a reference beam to generate an interference pattern therebetween and, then, controls the interference pattern to be stored in the storage medium such as an optical refractive crystal, wherein the optical refractive crystal is a material which may react differently depending on an amplitude and a phase of the interference pattern. In case the incident angle, the amplitude or the phase of the reference beam is changed, hundreds or thousands of binary data may be superposedly recorded at one location on a page basis.
Referring to FIG. 1, there is shown a conventional holographic digital data storage system, wherein the conventional system comprises an encoding circuit 10, a recoding and reconstructing circuit 20 and a decoding circuit 30.
The encoding circuit 10 encodes input data to generate rectangular page data to be provided to the recording and reconstruction circuit 20, wherein the rectangular page data has an l×m pixel array, l and m being positive integers, respectively, and each pixel has a binary value to represent a pixel brightness.
The recoding and reconstruction circuit 20 has a laser 202, a beam splitter 204, a first and a second shutter 206 and 212, a first and a second mirror 208 and 216, an actuator 214, a spatial light modulator(SLM) 210, a storage medium 218 and a charge coupled device(CCD) 220. In the recording and reconstruction circuit 20, a laser beam emitted from the laser 202 has generally a shape of circle and impinges onto the beam splitter 204 which is capable of partially reflecting the laser beam to thereby obtain a reference and a signal beam, wherein the reference beam is a portion of the laser beam transmitted through the beam splitter 204 and the signal beam is a remaining portion of the laser beam reflected from the beam splitter 204. The reference beam enters into the storage medium 218 through a reference beam path (S1) after being reflected by the first mirror 216 controlled by the actuator 214. In the meantime, the signal beam enters into the SLM 210 which converts the signal beam impinged thereonto into a modulated signal beam through a signal beam path (S2) after being reflected by the second mirror 208, wherein the modulated signal beam carries data in the form of pages after passing through the SLM 210.
The modulated signal beam is focused on the storage medium 218. In the storage medium 218, the modulated signal beam interferes with the reference beam to thereby generate an interference pattern therebetween, wherein the interference pattern contains information on the modulated signal beam, i.e., information on the rectangular page data itself. The interference pattern is converted into an optical index pattern in the storage medium 218 whose refractive index changes in response to the interference pattern to thereby record the modulated signal beam into the storage medium 218.
To read the modulated signal beam recorded in the storage medium 218, only the first shutter 212 remains open while the second shutter 206 turns to be closed so that the reference signal may be introduced to the storage medium 218 as a reconstruction reference beam. If the reconstruction reference beam is introduced into the storage medium 218, a retrieved signal beam corresponding to the optical index pattern may be generated from the storage medium 218, wherein the retrieved signal beam is generated by diffraction of the reconstruction reference beam from the storage medium 218. The retrieved signal beam enters the CCD 220 which is capable of detecting the power of the retrieved signal beam to reconstruct the rectangular modulated page data. The reconstructed rectangular modulated page data is transmitted to the decoding circuit 30.
The decoding circuit 30 decodes the reconstructed rectangular modulated page data to generate output data.
In general, the rectangular page data have been used to modulate the signal beam so that the modulated signal beam may have a shape of rectangle which is equal to that of a displaying device. However, a Gaussian distribution of a circular laser beam and the lens deformation cause the rectangular page data to be deformed in proportion to a distance from the center of each page. Accordingly, the peripheral region of the rectangular page data may have been considerably damaged and, especially, the 4 edges of the rectangular page data may have been seriously defected.